1. Field of the Invention
The present invention relates generally to monitor circuits. More specifically, the present invention relates to a device and method for the fault protection of monitor circuits.
2. State of the Art
Monitor circuits are utilized to provide an indication that a predetermined event has occurred. For example, monitor circuits may be included in an alarm system for providing a signal to a controller within the alarm system when an event trigger is detected.
For purposes of discussion, FIG. 1 shows an exemplary monitor circuit 10 configured to detect an external connection between input nodes 12 and 14 at the closing of a switch. As will be appreciated by one of ordinary skill in the art, such an event is known in the art as a “dry contact” or “dry connection” because no external power source is applied to either input node 12 or 14. In other words, in the example shown, signals ALARM+ and ALARM COM are coupled externally. Monitor circuit 10, as shown, further comprises diodes 16, 18, resistive element 20 and an exemplary isolated output section 34. Isolated output section 34 includes resistive element 22 and optically coupled isolator 24. Isolator 24 comprises light-emitting diode (“LED”) 26 and phototransistor 28. For purposes of this example, resistive element 20 may be approximately 620 Ohms and resistive element 22 may be approximately 20,000 Ohms. As shown in FIG. 1, input node 14 is coupled to ground. Input node 12 is coupled to both the anode of diode 16 and the cathode of LED 26. The cathode of diode 16 is coupled to the anode of LED 26 and to a 12 Vdc signal through a series connection of diode 18 and resistive element 20. The emitter of phototransistor 28 is coupled to ground and its collector is coupled to a 5 Vdc signal through resistive element 22. The collector of phototransistor 28 is coupled to resistive element 22 at output node 30 which produces an output signal (e.g., “Alarm_in”).
Isolator 24 and diodes 16, 18 provide protection to monitor circuit 10 as well as external circuitry (not shown.) Optically coupled isolator 24 provides electrical isolation between external event circuitry (not shown) coupled to input nodes 12, 14 and an external monitoring system (not shown) coupled to output node 30. Diode 16 protects LED 26 from any negative voltage spike that may occur at input node 12. Diode 18 prevents current from being drawn from external circuitry coupled to input node 12 in the event that the power source providing the 12 Vdc signal fails.
When input nodes 12 and 14 have been externally short-circuited, an electrical path is created between the 12 Vdc signal and ground through diode 18, resistive element 20 and LED 26. Thus, approximately 16 mA of current flows through LED 26, turning on phototransistor 28 and clamping resistive element 22 to ground. The ground at output node 30 indicates that the short-circuit event has occurred. Conversely, when input nodes 12 and 14 have not been short-circuited, current cannot flow through LED 26, phototransistor 28 is not turned on and output node 30 remains at 5 Vdc.
Monitor circuit 10 may be used, for example, in a fire alarm system that closes a dry contact switch and creates a connection between input nodes 12 and 14 when a fire is indicated by an appropriate signal. However, not all fire alarm systems, or circuitry associated therewith, are configured exactly the same. For example, some fire alarm systems may provide a 12 Vdc, 24 Vdc, 28 Vac or 120 Vac signal rather than a dry contact when a fire is sensed. Due to the diverse ways alarm systems may be configured, they can often be installed incorrectly, resulting in a fault condition arising within the monitor circuit 10.
As used herein, a “fault condition” may include an overvoltage condition or overcurrent condition which may cause harm to circuit elements. Further, a fault condition may include, for example, a signal which is provided as an alternating-current or “AC” when a direct-current “DC” signal is desired or expected. As will be recognized by those of ordinary skill in the art, an AC signal has periodic oscillations in which there will be a positive half cycle and a negative half cycle in each period.
Thus, for example, and referring back to FIG. 1, a fault condition may be present in the form of an overvoltage signal at input nodes 12, 14. Such an overvoltage signal can cause damage to monitor circuit 10 and result in incorrect operation of the alarm system. For example, if a 120 Vac signal is applied to input node 12, diode 18 will conduct on the negative half-cycle of the signal. The current flowing through diode 18, resistive element 20 and LED 26 will only be limited by the value of resistive element 20 (e.g., approximately 620 Ohms in the current example) and may destroy the circuit traces and LED 26, leaving the circuit inoperable. Such damage is costly and may cause additional harm to life and property if the faulty connection is not discovered before the presence of fire.
Thus, in view of the shortcomings in the art, it would be advantageous to develop a technique and device for protecting monitor circuits from fault conditions. It would also be advantageous to develop a technique and device to provide indication of the presence of a fault condition so the same could be appropriately remedied.